Abstract

The elevated temperature interfacial properties of continuous SiC fiber reinforced Ti matrix composites, primarily SCS-6/Timetal-21 S, have been examined in the tangential and normal direction in this experimental/numerical study. An elevated temperature fiber pushout apparatus has been built and used to test the interfacial shear strength and frictional shear stress at various temperatures. It was found that as temperature increases, tangential interface properties decrease, but the debonding behavior changes for test temperatures above 400°C. Additionally, transverse loading tests have shown a similar decrease in normal separation stress with increasing temperature. It was found that thermal exposure in vacuum for temperatures up to 650°C results in no noticeable increase in the interphase size, composition, or interphase strength properties. Aging in air causes a degradation of the interphase, resulting in deterioration of interphase properties, particularly at higher temperatures. The effect of variation of processing conditions, including variation of fiber volume fraction and of the time-temperature-pressure profile during consolidation, has been examined using concentric cylinder numerical model, and compared to experimental test results. Results show that the processing conditions required for full consolidation allow little variation in the composite stress state. An increase in fiber volume fraction increases the interfacial shear stress at all temperatures, with the most noticeable difference at room temperature, as well as a change in the characteristics of fiber pushout curves. Using interface properties, composite failure maps have been established for this material that describe the dominant failure mode as a function of temperature and stress intensity factor during loading. For reasonable use temperatures, the boundary between fiber bridging and interface debonding occurs at decreasing Kmax with increasing temperature. Interface debonding behavior, in terms of the cohesive zone model parameters of maximum traction, maximum separation and cohesive energy, in both the normal and tangential direction, has been established for the MMC composite system SCS-6/Timetal-21 S, for temperatures from 23 to 650°C. The numerical model yields good agreement with experimental results. For both normal and tangential decohesion, the maximum traction, Tmax, decreases with increasing temperature, while the characteristic length, δ, and work of fracture, &phis;, increase with increasing temperature. ^